MR13A-0987
Porous Compaction in Transient Creep Regime and Implications for Melt and Petroleum Extraction
Liquid segregation through a porous medium depends on the ability of the matrix to deform and compact. The key question of the rheological behavior of the porous matrix has been mainly investigated for Newtonian and simple visco-elastic Maxwell-like rheologies. Earth's materials have a more complex rheological behavior, in which the balance between the elastic and viscous contribution to the deformation is time-dependent. In this paper, we propose a Burger-type model to investigate the implications of transient rheology for viscous compaction of a porous material. The model is characterized by three dimensionless parameters: (1) the Deborah number, De, defined as the ratio of an elastic time scale over the compaction time scale, (2) the ratio of the transient and steady viscosities, λμ, and (3) the ratio of the transient and steady rigidity moduli, λG. We perform 1-D numerical experiments to illustrate the general behavior of compaction in a transient creep regime as a function of the model parameters. For small De (De< 10-2) the compaction occurs in the classical viscous mode and solitary waves (magmons) are generated. For larger De, the compaction mechanism is mainly controlled by λμ. For small transient viscosity (λμ → 0), compaction occurs in an elastic mode and shock waves are generated. For increasing λμ, two new regimes are observed, first "shaggy" shock waves and then "polytons". The threshold value of λμ that defines the two regimes is a decreasing function of De. Shaggy shock waves are characterized by the presence of secondary peaks at the wave propagation front. The length-scale of the peaks is a decreasing function of λG and their amplitude decreases along the propagation. In the polytons regime, the peaks tend to detach and mimic the behavior of solitary waves. Based on a discussion of the likely values of the model parameters for the Earth, we conclude that the influence of transient creep should not be neglected. Polytons and shaggy shock waves are expected both in the mantle and in sedimentary basins. Polytons will require a particular attention as they imply larger extraction velocities and smaller compaction length-scales than the usual magmons.
MR13A-0988
Partitioning of FeO between liquid Fe-Ni metal and magnesiowüstite at high pressures and temperatures
The Earth's outer core is believed to be composed of liquid Fe-Ni alloy together with one or more light elements of which oxygen is a possible candidate. Systematic investigations of the partitioning of oxygen between Fe-Ni liquid and silicates/oxides at high pressures and temperatures are scarce, although such studies have been performed on partitioning between Fe melt and silicates/oxides up to core-mantle boundary pressures (e.g. Takafuji et al. 2005; Sakai et al. 2006; Asahara et al. 2007). Their experimental results and thermodynamic interpretations show that the partitioning of FeO into Fe melt, in equilibrium with silicates/oxides, has a minimum value at a pressure of 15-20 GPa (Asahara et al. 2007). O'Neill et al. (1998) and Rubie et al. (2004) showed that the partitioning of FeO into Fe-Ni melt in equilibrium with magnesiowüstite decreases in the pressure range of 5- 23 GPa. In this study, we have investigated the partition coefficient of FeO between Fe-Ni melt and magnesiowüstite (Kd=XmetFeXmetO/XmvFeO) at 5-24.5 GPa and 2473-2873 K in order to clarify the effect of Ni on the partitioning of FeO into Fe liquid. High-pressure experiments were performed using a multianvil apparatus at the Bayerisches Geoinstitut. Starting metallic compositions were Fe-10 wt.% Ni and Ni-free Fe to which 0.5-1.5 wt.% O were added. Both compositions were run simultaneously in each experiment by containing them in a double-chamber MgO capsule. Our experimental results show that the Kd for Fe-10 wt.% Ni melt + magnesiowüstite is lower than for Fe melt + magnesiowüstite. The value of Kd for Fe-Ni melt + magnesiowüstite increases with temperature, but the pressure dependence is not clearly defined by our results. Based on this study, the concentration of oxygen in the Earth's core could be lower than previously estimated, although the effect of sulfur on oxygen solubility still needs to be investigated systematically.
MR13A-0989
Melting behavior of (Mg,Fe)O solid solutions at high pressure
A new multi-anvil cell assembly was developed to provide stable heating up to 3473 K at pressures below 7 GPa. We have performed a series of experiments to investigate both the compositional and pressure effects on the melting behavior of (Mg,Fe)O solid solutions up to 3373 K at pressures of 3, 5 and 7 GPa in the multi-anvil apparatus. The occurrence of the partial melting is identified by the separation of FeO-rich partial melt from large MgO-rich (Mg,Fe)O crystals, with Mg-numbers (Mg*=100MgO/(MgO+FeO)) between 6 and 95 in the quenched phases. The temperature at which the partial melting of (Mg,Fe)O occurs increases slowly with increasing MgO content in the FeO-rich portion, from 2073 K for (Mg,Fe)O with Mg*=34 to 2273 K for Mg*=76 at 3 GPa, whereas the slope becomes much steeper in the MgO-rich portion, from 2773 K for Mg*=83 to 3373 K for Mg*=95. The experimental data at pressures of 3, 5 and 7 GPa indicate that the melting slope as a function of pressure is smaller in the FeO-rich portion (Mg*<75) than that in the MgO-rich portion. The melting temperature difference is ~100 K from 5 to 7 GPa in the FeO-rich portion, but this value increases to ~200 K in the MgO-rich portion, implying that MgO has a steeper melting curve than FeO end-member. The topology of the MgO-FeO system implies that our melting slope dTm/dP of MgO at zero pressure is several times higher than the value derived from previous experimental measurements of MgO melting at high pressure in a CO2 laser-heated diamond cell, but more consistent with some theoretically predicted values. Our results provide fundamental information for understanding deep melting in the Earth's interior, in particular, the presence of partial melt in the "ultralow velocity zone (ULVZ)" at the base of the lower mantle.
MR13A-0990
Investigation of Fe-FeS phase diagram and liquid structure at high pressure and high temperature
Sulfur is believed to be an alloying light element in iron-rich planetary cores such as those of the Earth and Mars 1, 2. Recent studies have suggested that Mars, like the Earth, could have a liquid metallic outer core together with a solid inner core 3. Hence, it is important to investigate the evolution of the Fe-FeS phase diagram and of the structural properties of the liquid Fe-FeS alloys in respect to pressure, temperature and sulphur content. A new cell assembly has been developed to heat samples to more than 1300 K at 17 GPa using the Paris Edinburgh Press4. This allows us to conduct detailed structural investigations of the Fe-FeS eutectic liquid by in situ X-ray diffraction5 . Analysis of these data highlights an increase of the liquid compacity with increasing pressure. We also show that the eutectic liquid structure is closer to that of FeSi, explaining the closure of the miscibility gap in the Fe-S-Si system 6. The evolution of the Fe-FeS eutectic liquid structure at high pressure could have significant effect on extrapolated wave speed of metallic Fe-FeS alloy at core pressures. We have used a double-sided laser-heated diamond-anvil cell 7 to study the Fe-FeS phase diagram up to 65 GPa and 2500 K8. We used laser heated diamond anvil cell coupled with synchrotron radiation and confirm a S- solubility below 4 at% (2.3 %wt) up to 65 GPa. The eutectic temperatures present a uniform increase, with a rate of ~15K/GPa, up to 65 GPa and 2200 K. Finally, we present new constraints on the phase diagram evolution to very high pressures which provide unambiguous evidence for an upper limit of 4-8 %wt for the inner core S- content. Therefore, sulphur is not favoured to be the major light element in the Earth's core. 1. Allegre, C. J., Poirier, J. P., Humler, E. & Hofmann, A. W. The chemical composition of the Earth. Earth Planet. Sc. Lett. 134, 515-526 (1995). 2. Sohl, F. & Spohn, T. The interior structure of Mars : Implications from SNC meteorites. J. Geophys. Res. 102, 1613-1635 (1997). 3. Yoder, C. F., Konopliv, A. S., Yuan, D. N., Standish, E. M. & Folkner, W. M. Fluid core size of Mars from detection of the solar tide. Science 300, 299-303 (2003). 4. Morard, G. et al. Optimization of Paris Edinburgh cell assemblies for in situ monochromatic X-ray diffraction and X-ray absorption. High Press. Res. 27, 1-11 (2007). 5. Morard, G. et al. Structure of eutectic Fe-FeS melts up to 17 GPa: Implications for planetary cores. Earth Planet. Sc. Lett. in press (2007). 6. Sanloup, C. & Fei, Y. Closure of the Fe-S-Si liquid miscibility gap at high pressure. Phys. Earth Plan. Int. 147, 57 (2004). 7. Mezouar, M. et al. Development of a new state-of-the-art beamline optimized for monochromatic single crystal and powder X-ray diffraction under extreme conditions at the ESRF. J. Synch. Rad. 12, 659-664 (2005). 8. Morard, G. et al. Experimental constraints on the Earth's core sulphur content. Nature (Submitted).
MR13A-0991
Melting in the Fe-Si System at High Pressures and Temperatures
It has long been known that the Earth's core consists mainly of iron and nickel with several weight percent of a light element. Geochemical and cosmochemical constraints suggest Si as a strong candidate for this light element contribution. Therefore it is important to understand the phase diagram, including melting relations, in the Fe-Si binary at high pressures and temperatures. We have conducted melting experiments in the Fe-Si system using double-sided laser heating in diamond anvil cells. Temperatures were measured using standard spectroradiometric techniques, and also using a new system for obtaining 2D temperature distributions over the laser heated spots. Melting was established by a variety of criteria, including: analysis of temperature, laser power and emissivity data during heating; optical examination; and electron microscopy of the recovered samples. Electron microprobe analysis of recovered samples also indicated the direction of partitioning of Si between solid and liquid. Our melting temperatures are comparable to earlier results obtained from multi-anvil press experiments.
MR13A-0992
The effects of temperature and light elements on the interfacial tension of liquid iron under high pressure
Interfacial tension is one of the most important properties of liquid iron-alloy to control the core formation process in the Earth and planets. The aim of this study is to clarify the effects of temperature and light elements (S and P) on the interfation tension of liquid iron under high pressure. In this study, high pressure in-situ measurement of interfacial tension was carried out for the liquid Fe-S and Fe-P alloys using sessile drop method combined with X-ray radiography technique up to 2 GPa and 1923 K. The sessile drop method has been widely used for measurement of surface and interfacial tensions at ambient pressure. The effect of temperature on the interfacial tension for both Fe-S and Fe-P liquids is quite small in the range of measurement (1623 – 1923 K). The interfacial tension of liquid Fe-S decreases linearly from 802 to 112 mN/m with increasing sulphur content of 0 - 40 at%. Thus, sulphur reduces significantly the interfacial tension of liquid iron. On the other hand, phosphorus does not affect to the interfacial tension of liquid iron. These tendencies are in good agreement with the data measured at ambient pressure. Therefore, the behaviour of light elements on the interface at ambient pressure is maintained at least up to 2 GPa and the effect of light elements on the interfacial tension of liquid iron depends on the element species. The shape change of the liquid Fe-S was observed before and after quenching. This suggests that the in-situ measurement at high pressure and temperature is essential in order to obtain the true interfacial tension, i.e. interconnectivity of liquid iron-alloy.
MR13A-0993
Melting of Iron – Light-Element Alloys in the Laser Heated DAC
Seismic data indicate that the Earth's outer core is ~10% less dense than pure iron at the pertinent conditions of pressure and temperature based on the experimentally determined equation of state of iron (Shanker et al., 2004). This core density deficit can be explained by the presence of a light element component such as H, C, N, O, S or Si, or a mixture of these. Constraints on core composition, phase relations and temperature can be derived from knowledge of the melting relations in relevant iron – light-element systems using a thermodynamic approach coupled with observations from seismology (Helffrich & Kaneshima, 2004). Here we make a progress report on our systematic effort to determine melting phase relations in binary Fe-alloy systems at high pressures using laser-heated DAC techniques. Foils of compressed powder or pre-fabricated chips of Fe alloys are loaded into ~100 micron holes in pre- indented stainless steel or rhenium gaskets. We use a variety of pressure media including sapphire, ruby, alumina gel, NaCl, and Argon, which also serve as thermal insulators. Pressures are measured before and after experiments using the fluorescence shift of ruby. Samples are heated using a 60W Nd:YLF laser with a double- sided heating geometry, and temperatures are measured using standard spectro-radiometric techniques (Walter & Koga, 2004). Melting is deduced from sudden, obvious and repeatable discontinuities in the temperature and emissivity vs. laser power functions as expected from invariant melting. In some cases clear visual observation of melt motion is coincident with these discontinuities. Our results to date show good correspondence with previous measurements where data overlap for Fe, Pt, the Fe-S eutectic, the Fe-Fe3C eutectic and Fe3C. Our melting curve for Fe3C up to ~75 GPa is considerably lower in temperature at high pressures than previously predicted (Wood, 1993), yielding an extrapolated temperature of about 4000 K at the core mantle boundary using a Simon fit to the data. We predict a singular point along the Fe3C melting curve at ~ 20 GPa where congruent melting begins, and possibly another singular point at ~ 70 GPa where the Fe-Fe3C eutectic may intersect the Fe3C liquidus indicating that the eutectic composition has risen to become equal to that of Fe3C. We will present these and other new results.
MR13A-0994
Modes of Planetary Reshaping During Core Formation: Numerical Study
The early stages of terrestrial planetary accretion and differentiation related to core formation are largely enigmatic and require extensive realistic numerical modelling efforts especially in 2D(a cross-section of a spherical planet) and 3D geometries. One early stage of terrestrial planets was assumed to have a gravitationally unstable three-layer structure, the innermost undifferentiated solid core, the intermediate metal-melt layer, and the outermost silicate-melt layer, which leads to a Rayleigh-Taylor instability of various orders. We have developed a 2D thermomechanical numerical model for studying core formation in a self-gravitating planetary body surrounded by mass-less weak medium by using a combination of finite-differences with a Lagrangian marker-in-cell technique on a fully staggered Cartesian grid. We include a free planetary surface, spontaneously evolving gravity field, visco(elasto)plastic rheology of materials and feedback from shear heating. Benchmarking of this novel numerical method against available analytical solutions (Ida et al., 1987, Earth Moon Planets, 44, 149-174) has demonstrated high accuracy of the numerical results in the non inertial reshaping regime. Assuming the three-layered model (primordial protocore, metal and silicate layers) we investigated the influence of the viscosity contrast between the layers on the geometrical mode of planetary reshaping. In contrast to a previously conducted numerical study (Honda et al., 1993, JGR, 98, 2075-2089) we explored a broad range of viscosity ratios between the metallic layer and the protocore (0.001-1000) as well as between the silicate layer and the protocore (0.001-1000). A new important prediction from our study is that realistic modes of planetary reshaping characterized by a high viscosity contrast between the cold protocore and hot molten silicate layer always results in the transient exposure of the prorotocore to the planetary surface during the early stages of core formation. This causes large lateral variations in the thickness of a global magma ocean (which should have major geochemical consequences) and creates possibility of destruction and reworking of the exposed protocore by ongoing planetesimal impacts. The dominant L=1 nature of this instability may explain the crustal dichotomy on planets such as Mars.